Method of controlled air seasoning of wood



April 19, 1955 J. A. VAUGHAN 2,706,344

METHOD oF coNTRoLLED AIR sEAsoNING oF woon Filed March 11, 1953 2Sheets-Sheet l JosEPH A. vAsHAN ATTORNEYS April 19 1955 J. A. VAUGHAN2,706,344

METHOD oF OONTROLLED AIR sEAsoNING OF woon ZONE A INVENTR JOSEPH AA.vVALIGHAN F is. a.

BY @mfw ATTORNEYS United States Patent O METHOD OF CONTROLLED AIRSEASONING F WOOD Joseph A. Vaughan, Atlanta, Ga., assignor to SouthernWood Preserving Company, Atlanta, Ga., a corporation of GeorgiaApplication March 11, 1953, Serial No. 341,635

4 Claims. (Cl. 34-31) This invention relates to a method of airseasoning wood, such a green crossties, poles, crossarrns, and lumber,to reduce the moisture content to a point suitable for subsequentpreservative treatment or to reduce the subsequent shipping weight.

This application relates to certain improvements in the method disclosedin my application, Ser, No. 297,925, tiled July 9, 1952, for Method ofControlling Air Seasoning of Wood.

The main object of the invention is to provide a greatly improved andeconomical method of air seasoning wood, as above set forth, to reducethe time required for drying to a fraction of the time usually requiredfor seasoning in the open air and to eliminate deterioration or degrade,due to abnormal checking and decay. The idea is to reduce the dryingtime by so controlling drying conditions as to avoid such deterioration.

Another object of the invention is to provide a relatively simple andpractical method of drying wood in a tunnel drier, using a stream ofatmospheric air heated to a sub- Stantially constant optimum temperatureand controlling the drying temperature at different locations or zonesin the tunnel in response to changes in the temperature of the outboundair.

A further object of the invention is to provide an economical method ofair seasoning wood in a long drying tunnel by maintaining thetemperature of the inbound air stream at a predetermined dryingtemperature and so controlling the temperature conditions in the tunnelthat the temperature of the outbound air stream will be maintained at apredetermined minimum which will insure initial drying of charges ofwood introduced into the tunnel at the outbound air steam end. The ideais to obtain a maximum heating eiciency by preventing the moisture ladenair from being discharged while it still has some useful dryingcapacity.

Still another object of the invention is to provide a most economicaland effective method of air seasoning charges of wood in a long tunnelby delivering fresh air to the tunnel at different zones along thetunnel in response to an abnormal or predetermined increase in thetemperature of the outbound air stream. The idea is to maintain adesirable rate of drying of a charge of wood in a tunnel of any givenlength, regardless of the initial moisture content of the wood, thusaccommodating the length of the tunnel to a particular charge of wood.

Another object of the invention is to provide a method of air seasoningwood, as above set forth, which makes it possible to employ relativelysimple automatic control equipment.

A further object of the invention is to provide a method of airseasoning wood in a tunnel, as above set forth, which makes it possibleto vary the velocity of the drying air in different parts of the tunnelso as to obtain the most efficient use of the contained heat and toeffect the desired rate of drying.

Other objects and advantages will appear in the following specification,when considered in connection with the accompanying drawings, wherein: p

Fig. l is a diagrammatic plan view of a tunnel drier adapted to practicethe method;

Fig. 2 is a vertical sectional view of the drier shown in Fig. 1; and

Fig. 3 is a simplified wiring diagram of electrical control apparatussuitable for practicing the method.

As stated in my aforesaid copending application, the method ofcontrolling air seasoning is designed to reduce 2,706,344 Patented Apr.1.9, 1955 the time normally required for seasoning wood in the open airand to eliminate deterioration and degrade, due to abnormal checking,decay and the like, which may occur during natural seasoning. Thepresent method provides important improvements in the operation oftunnel driers to dry charges of wood most eiectively.

In accordance with this invention, the drying air is delivered orintroduced into a drying tunnel, preferably in the form of an insulatedbuilding 10. While the building may have two or more parallel tunnels toprovide a multirun unit, it is shown as having a single tunnel.

The volume of wood to be dried and the air volume and velocity desired,are factors which influence the dimensions of the tunnel. For dryingcrossties, poles, and the like, the tunnel may be ten to fifteen feetsquare in cross section and about three hundred to seven hundred feetlong, to permit successive charges or a number of stacks of thecrossties, poles, or other materials, to be conveyed through the tunnel.

The tunnel is shown as having suitable doors 1 1 at its opposite endsand the charges of wood, in the form of stacks 12, are preferably loadedon trams 13 conveyed through the tunnel on tracks 14. The pieces in thestacks are spaced apart to permit free passage or circulation of dryingair in contact with all of them. The construction and arrangement aresuch that loaded trams of green wood will be introduced into the tunnelin succession as tram load of dried material are removed, so that thedrying process is continuous.

Drying air is introduced at a substantially constant rate into thetunnel at one end through a conduit 15, shown as being on top of thetunnel, and is discharged from the other end through an exit conduit 16,An exhaust fan 17 is shown in the exit conduit to cause induced draft ofthe drying air through the tunnel. However, it is contemplated that aforced draft fan may be employed.

In accordance with the invention, the drying air is preheated to anoptimum constant temperature of F. to F., in a heater 18 of any suitabletype. For the purpose of illustration, burners 19 are shown as beingsupplied with gas by a feed pipe 20 having a valve 21 controlled by aproportioning motor 22.

The heated, inbound air introduced into the delivery end or zone D ismaintained at a constant temperature by a thermo-couple 23 located inthe delivery zone D and a potentiometer 24 of the Minneapolis-Honeywelltype connected to control the motor 22, in a manner well known in theart.

In accordance with this invention, the temperature of the outbound airstream in the exit zone E is maintained constant at between above 70 F.and 80 F., by either introducing-fresh cool air at different zones inthe tunnel to reduce the temperature of the air stream, or by boostingthe temperature of the air stream about midway of the length of thetunnel, both in response to variations in the temperature of the airstream in zone E. In the illustrated embodiment, fresh air is adapted tobe admitted into the top of the tunnel to separated zones A, B and C,through conduits 25, 26 and 27 carrying dampers 28, 29 and 30,respectively, preferably in the form of louvers. In the present example,the zone A is located about midway of the length of the tunnel and thezones B and C are about two-thirds and five-sixths of the length of thetunnel from the delivery zone D. The location of these zones is selectedso that the temperature of the drying air may be controlled as ittravels through at least half the length of the tunnel before it reachesthe exit end.

The dampers 28, 29 and 30 in the fresh air conduits are operated bymodutrol motors 31, 32 and 33, respectively. The motors are controlledby potentiometers 34, 35 and 36, respectively, connected tothermo-couples 37, 38 and 39, located in the zones A, B and E,respectively. The arrangement is such that the potentiometers can be setto maintain constant and predetermined temperatures of the drying air inthe separate zones. The temperature in the Zone A is set between 100 F.and 125 F., in zone B, between 90 F. and 100 F., and in Zone C, between70 F. and 80 F. In the last zone C, the temperature is maintained thesame as that of the outbound air in zone E. These temperatures have beenfound to be most effective.

When the dampers in the ducts or conduits leading to Zones A, B and Care opened, fresh, unheated air is admitted through the top of thetunnel and mixes with the drying air stream to reduce its temperatureand maintain the set temperature conditions in the respective zones.This results in a slight increase in velocity of the air stream, becauseof the reduction in the resistance to flow through the entire charge.The volume and velocity of the drying air are correspondingly reducedbut without materially reducing the drying capacity of the heated air.Thus, substantially all of the sensible heat in the drying air isutilized.

In warm climates, where the atmospheric temperature on sunny days duringthe summer months is above that set for the outbound air, the dampers inall of the air ducts 25, 26 and 27 will be opened to admit as much warm,fresh air as possible and thereby reduce the fuel consumption andincrease the eliiciency of the drier.

In cold climates, or during winter months, when the atmospherictemperature is far below that set for the outbound air, it will benecessary to boost the temperature of the drying air about midway of thelength of the tunnel, say, at zone A. For that purpose, a proportioningbooster heat 40 having a burner 41 supplied with gas from the gas line20, is located on one side of the tunnel to discharge heated air into aduct 42 directed across the bottom, so that the heated stream of airwill rise and mix with the partially cooled air stream from the deliveryzone D.

The operation of the booster heater' is shown as being v controlled inresponse to a decrease in the temperature of the outbound air stream.For that purpose, a thermocouple 43 is located in Zone E and isconnected to a potentiometer or regulator 44 in the circuit to the motor45 which controls a gas valve 46. The regulator 44 is also set at thedesired temperature of the outbound air stream, or between 70 F. and 80F.

The circuits to the potentiometers or regulators for the damper motors31, 32 and 33, and for the booster heater motor 45, are made and brokenby a master circuit controller 47, which includes a double-throw,double-pole mercoid switch 48 of a well known type operated by athermo-couple 49 in the outbound air duct 16. This controller orregulator is set to tilt the switch and close the circuits leading tothe potentiometers 34, 35 and 36 which control the damper motors whenthe temperature in the outbound air stream ranges between, say, about 76F. and 80 F., which is above the set temperature of the outbound air. Itis also set to tilt the switch in the opposite direction to close thecircuit to the potentiometer 44 which controls the booster heater motorwhen the temperature of the outbound air stream ranges between, say 70F. and 74 F., which is below the desired temperature of the outboundair.

In Fig. 3, allof the motors and their regulators are shown as beingconnected to a common source of current and they are located in thezones depicted by heavy dot and dash lines. Incidentally, zone A isshown twice for convenience in illustrating the circuits. The firstfresh air duct midway of the length of the tunnel and the booster heaterare described as being located in that zone.

In practicing the method, stacks of the green wood are loaded on thetrams and conveyed periodically and in succession through the tunnel.The rate of charging and removing them is determined by the dryingschedule, which depends upon the species of the wood and the tinalmoisture content desired as well as the size of the pieces. The seasonedstacks are preferably removed one or more at a time each day, accordingto a predetermined time schedule, and the remaining stacks are advancedin the tunnel to make room for green stacks to take the places of theones removed. The stacks are preferably of such size that the crosssection of the wood is approximately sixty percent of the cross sectionof the tunnel and the air space between the pieces is about fortypercent of the cross section.

An illustrative example of the method will now be explained, aspracticed in a tunnel drier, say, 300 feet long and 13 feet square incross section. A drier of this size is adapted to be used for airconditioning crossties, poles and lumber.

Assume that gum crossties having an initial moisture content of about90% are to be dried to a final moisture content of about 45%, dryweight, for subsequent preservative treatment. Tram loads of suchcrossties are approximately 81/2 feet long, about 12 feet high, and 12feet wide. Spacing strips in the stacks provide an air space of about40% of the cross section of the tunnel.

The regulator 22 for the main air heater 18 is set to maintain thetemperature at about F. The regulator 34 for the air damper at zone A isset at 110 F.; the regulator 35 for the damper at zone B is set at 95F.; and the regulator 36 for the damper at zone C is set at 75 F., whichis the desired temperature of the outbound air stream to provide foreffective initial drying. The regulator 44 in zone E is also set at 75F. to control the operation 0f the booster heat 40.

The exhaust fan and main heater are started in operation and loadedtrams are conveyed into the tunnel, say, two at a time, on successivedays, until the tunnel is fully charged. ln this example, a full chargeis 34 tram loads. The drying time or cycle is determined by the volumeand velocity of the drying air. In the present example, the exhaust fanis operated at a capacity of about 40,000 cubic feet per minute, causingthe air ow through the stacks of wood at a velocity of about 575 feetper minute, so that the crossties will be dried to the required moisturecontent in approximately 17 days without damage, due to checking. Thisrate of drying such crossties has been found to be very effective. Whenthe time cycle is thus established, two tram loads will be removed andtwo loads added every day, so that the operation will be continuous. Ofcourse, the schedule can be changed according to time required to seasondifferent species of wood, and/or types of pieces to be dried.

When the temperature of the outbound air stream rises to from 76 to 80F., the master mercoid switch 47 will close the Circuits leading to allof the damper motors, so that their regulators can control them ashereinbefore described, to maintan substantially constant predeterminedtemperatures in the zones A, B and C, and thus lower the temperature ofthe outbound air stream. When the temperature of the outbound air streamfalls to 70 F., or below, the mercoid switch will close the circuit tothe booster heater motor 45, so that it can be controlled by itsregulator and thus raise the temperature of the outbound air stream to75 F.

The simplified method of control is entirely automatic so that the drierdoes not require a full time attendant. Furthermore, this method enablesthe drier to use substantially all of the effective heat contained inthe drying air, thereby insuring the most economical use of the fuelconsumed. Also, the method makes it possible to maintain substantiallyideal drying conditions throughout the length of a drier.

Obviously, the invention is not restricted to the particular embodimentthereof herein shown and described.

What is claimed is:

l. The method of air seasoning green wood, which is characterized byperiodically charging substantially uniform stacks of the wood into along tunnel and conveying them therethrough; introducing heated air at asubstantially constant temperature between about 120 F. and F. into thetunnel at the stack outlet end and discharging it from the tunnel at thestack charging end; and changing the temperature of the air streamintermediate the ends of the tunnel in response to variations in thetemperature of the outbound air to maintain a predetermined progressivereduction in the temperature of said air stream so that the temperatureof the outbound aiwill be maintained between about 70 F. and about 2.The method of air seasoning green wood, as set forth in claim l, whereinoutside air, in response t0 variations in the the temperature of theoutbound air, is automatically introduced intermediate the ends of thetunnel to mix with the air stream and control its temperature.

3. The method of air seasoning green wood, as set forth in claim l,wherein the temperature of the air stream intermediate the ends of thetunnel is changed by introducing outside air into a plurality of spacedzones in the tunnel with the irst zone located near the middle of thetunnel and the other Zones between it and the stack charging end, and bycontrolling the amount of outside air admitted to the respective zonesso as to maintain substantially constant and eicient predeterminedtemperatures therein to insure proper drying conditions in each zone.

4. The method of air seasoning green wood, which is characterized byperiodically charging substantially uniform stacks of wood into a longtunnel and conveying them therethrough; introducing heated air at asubstantially constant temperature between about 120 F. and 160 F. intothe tunnel at the stack outlet end and discharging it from the tunnel atthe stack charging end; and changing the temperature of the air streamintermediate the ends of the tunnel in accordance with variations in thetemperature of the outbound air to maintain a predetermined progressivereduction in the temperature of said air stream so that the temperatureof the outbound zi'owfirll be maintained between about 70 F. and aboutReferences Cited in the le of this patent UNITED STATES PATENTS Re.14,528 Andrews Oct. 8, 1918 6 440,451 Van Duzer Nov. 11, 1890 923,092Valentine May 25, 1909 1,353,591 Hope etal Sept. 21, 1920 1,533,715Tiemann Apr. 14, 1925 1,539,230 Anderson May 26, 1925 1,567,023 LarssonDec. 22, 1925 2,152,312 Jennings et al Mar. 28, 1939 OTHER REFERENCESAccelerated Kiln Schedules, July 1951. Publication of U. S. D. A.,Forest Products Laboratory, Madison, Wisconsin, Report No. D1901, 15pages, pages 6 and 7 relied on.

